In recent years, liquid crystal panels and organic electroluminescent (EL) panels are used in commercially available display devices such as liquid crystal monitors, liquid crystal televisions, feature phones, smartphones, tablets, electronic books, and notebook personal computers in terms of eco-friendliness such as space saving or electric power saving. Such a display device includes a pair of substrates which are disposed so as to interleave an optical element layer and a protection member such as a cover glass located on the outer surface of the substrate which is disposed in the viewer side.
Recently, there is strong need for highly thin display devices. To achieve such highly thin display devices, there is a technique to perform mechanical polishing or chemical polishing using, for example, hydrofluoric acid to the pair of substrates opposed to each other.